Fabrication method for composite structure adapted for controlled structural deformation

ABSTRACT

A composite structure and related fabrication method is provided which has a plurality of composite material layers and at least one embedded shape memory alloy component for providing controlled structural deformation. The shape memory alloy component includes a shape memory alloy tendon having a relaxed shape at temperatures below a predetermined transition temperature and a contracted shape at temperatures above the predetermined transition temperature. The shape memory alloy component also comprises a pair of electrically insulating opposing face sheets adhered to opposite sides of the shape memory alloy tendon to electrically isolate the shape memory alloy tendon from the surrounding composite material layers. The shape memory alloy component is embedded within the plurality of composite material layers such that raising the temperature of the shape memory alloy tendon above the transition temperature creates a controlled structural deformation of both the shape memory alloy component and the surrounding composite material layers.

GOVERNMENT RIGHTS

The United States Government may have rights in this invention pursuantto ARPA Agreement No. MDA 972-93-2-0010 awarded by the Advanced ResearchProjects Agency.

FIELD OF THE INVENTION

The present invention relates generally to composite structures andassociated fabrication methods and, more particularly, to compositestructures adapted for controlled structural deformation and associatedfabrication methods.

BACKGROUND OF THE INVENTION

In many industries wherein weight is a limiting performance parametersuch as in the aircraft or automobile industries, reinforced compositematerials are fast becoming a light-weight and cost effectivealternative to conventional metallic materials such as aluminum,titanium, and the like. In high stress applications, composite materialsare typically formed of thermoplastic resins which are reinforced withfibers such as carbon. The carbon fibers act to strengthen, i.e.,"reinforce", the thermoplastic resin. Thus, the reinforced compositematerials have increased strength and are better able to withstandhigher temperatures and higher pressures than a non-reinforcedresin--all the while saving unwanted pounds in comparison withconventional structural materials.

Although composite materials have many advantages, they are not withoutproblems which typically include processing difficulties and materialdeficiencies introduced during the manufacturing process. These materialdeficiencies can include voids created during the fabrication of acomposite structure and other material deficiencies as a result of afiber/resin interruption. Regardless of the origin, materialdeficiencies can introduce serious performance problems.

Notwithstanding the criticality of minimizing irregularities in thecomposite materials, such as voids and fiber inclusions or the like, soas to maintain the integrity of the composite structure, compositecomponents can be manufactured in a variety of ways depending upon theparticular application of the composite component. For example, largeand complex composite structures can be fabricated by laying up orstacking a number of composite plies on an underlying tool or mandrelwhich, at least partially, defines the shape of the resulting compositestructure. The plies are thereafter consolidated by placing thestructure, under pressure, into an autoclave (or oven) to heat thematerial to a sufficient temperature and for a sufficient time to insureresin flow and bonding of the plies into an integral laminate compositestructure.

It is also known to those skilled in the art to form composite parts byemploying a fiber placement process. See, for example, Richard Sharp, etal., Selection/Fabrication Issues for Thermoplastic Material FiberPlacement, J. of Thermoplastic Composite Materials, Vol. 8, pp. 2-14(January 1995). According to a conventional fiber placement process, oneor more ribbons of composite material (also known as a composite tow)are laid down on a substrate. The substrate may be a tool or mandrel,but, more conventionally, is formed of one or more underlying layers ofcomposite material which have been previously laid down andconsolidated. A conventional fiber placement process utilizes a directedlaser heat source to consolidate the plies of composite material at alocalized nit point. In particular, the ribbon of composite material andthe underlying substrate are heated at the nit point to melt the resinand are simultaneously compacted to insure consolidation. For example,the ribbons of composite material can be compacted by a compliantpressure roller as described by U.S. Pat. No. 5,058,497 to James C.Bishop, et al. To complete the part, additional layers of compositematerial can be applied in a side-by-side manner and can be subjected tolocalized heat and pressure during the consolidation process. SeeMatthew M. Thomas et al., Manufacturing of Smart Structures Using FiberPlacement Manufacturing Processes, 2447 SPIE 266 (1995).

As described above, the nit point is held for a very short time atrelatively high temperatures and pressures. As such, the layered orstacked plies are successively exposed to relatively high temperaturesand pressures in order to sufficiently bond the plies of compositematerial into the integral laminate structure--without any need for apost-process autoclave step. Nevertheless, the consolidation ofcarbon-fiber reinforced plies by a fiber placement process typicallyrequires temperatures in excess of 1200° F. and high compactivepressures as great as 600-800 PSI.

Irrespective of the fabrication method, it is desirable in manyapplications for structural components, such as composite parts, toundergo controlled structural deformation. For example, the controlledstructural deformation of a structural component can dampen vibrationalresponses induced by external forces so as to control material fatiguein structures or can provide adaptive aircraft control surfaces. Thestructural deformations are conventionally created with mechanicalactuators, but these mechanical devices add system weight andcomplexity.

Alternatively, structural components can include shape memory alloy toperform mechanical work such as structural displacement and stiffening.Shape memory alloy is an alloy which has a first shape at temperaturesabove a predetermined transition temperature and a second shape attemperatures below the predetermined transition temperature. When theshape memory alloy is heated to the predetermined transitiontemperature, the shape memory alloy "remembers" its original shape andstiffens and contracts to its original heat-treated shape. Thus, if theshape memory alloy is embedded within a structural component, thestructural component will deform in a controlled or predictable manneras a result of the stiffening of the shape memory alloy upon beingheated to temperatures above the predetermined transition temperature.

In the past, shape memory alloy wires were individually embeddeddirectly within the composite materials in an attempt to providecontrolled structural deformation. Because carbon reinforced compositematerials are conductive, however, any heating of a shape memory alloywire could also heat other shape memory alloy wires which were inelectrical contact with the composite structure, thereby creatingsecondary and unwanted actuation of the other shape memory alloy wires.In order to control the actual deformation of the composite partoccasioned by actuation of a first shape memory alloy wire, the shapememory alloy wires must therefore be electrically and thermally isolatedfrom the remainder of the composite structure and from the remainder ofthe shape memory alloy wires.

In order to provide this electrical and thermal isolation, shape memoryalloy wires have been individually sheathed or clad with a dielectriccoating. This cladding process is typically time-consuming, hard tocontrol, labor intensive and generally not suitable for a manufacturingenvironment. In embodiments in which the shape memory alloy takes theform of a shape memory alloy wire, attempts to electrically andthermally isolate the shape memory alloy wire, such as by disposing theshape memory alloy wire within slip fit sheath insulation, could deformthe wire and thereby induce malfunctions in any related structuralmovement.

In addition, the wires, sheathing, and cladding were oftentimes notsufficiently robust to withstand the high pressures and temperaturestypically encountered in the consolidation phase of a compositefabrication process. Moreover, the embedding of shape memory alloydirectly into the composite material could create dimensional mismatcheswithin the composite plies which, in turn, could create voids within thecomposite material, thereby diminishing the structural integrity andstrength of the composite material. For example, voids may be createdaround shape memory alloy wires which are embedded within a compositestructure since the size and shape of the shape memory alloy wires donot match the size and shape of the composite plies. These dimensionalmismatches between the shape memory alloy and the composite plies andthe resulting decrease in structural integrity are particularly evidentin composite structures fabricated by a fiber placement process in whichthe shape memory alloy wires are typically much smaller than the ribbonsof composite material which are laid down to create the resultingcomposite structure.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a shapememory alloy component which can be efficiently embedded within acomposite structure in order to provide controlled structuraldeformation of the composite structure.

It is another object of the present invention to provide a compositestructure having a shape memory alloy component which can be reliablyisolated, both electrically and thermally, from the remainder of thecomposite structure.

It is a further object of the present invention to provide a compositestructure which includes a shape memory alloy component which does notimpair the structural integrity and strength of the composite structure.

It is yet another object of the present invention to provide an improvedmethod of fabricating a composite structure which is adapted forcontrolled structural deformation.

These and other objects are provided, according to the presentinvention, by a composite structure having a number of compositematerial layers and an embedded shape memory alloy component whichincludes a shape memory alloy tendon disposed between a pair ofelectrically insulating face sheets. The shape memory alloy component ispreferably embedded within the composite material layers such that theshape memory alloy tendon can be externally actuated. By actuating theshape memory alloy tendon, such as by raising the temperature of theshape memory alloy tendon above a predetermined transition temperature,the shape memory alloy tendon contracts to create a controlledstructural deformation of both the shape memory alloy component and atleast a portion of the composite structure.

The shape memory alloy tendon can have a variety of configurations, suchas a number of shape memory alloy wires or a shape memory alloy foil.Regardless of the particular configuration, the shape memory alloytendon is disposed between a pair of electrically insulating facesheets, such as a pair of face sheets formed of a thermoplasticmaterial, such as polyetheretherketone and polyetherimide, in order toeffectively isolate the shape memory alloy tendon from the remainder ofthe composite structure. The face sheets are preferably adhered toopposite sides of the shape memory alloy tendon to form an integralshape memory alloy component. In addition to adhering the face sheets tothe shape memory alloy tendon, the adhesive is also preferably disposedalong the opposed side edges of the shape memory alloy component tofurther electrically isolate the shape memory alloy tendon from theremainder of the composite structure.

According to the fabrication method of the present invention, the shapememory alloy component is formed and is subsequently embedded within aplurality of layers of composite material. In order to form the shapememory alloy component, a shape memory alloy tendon is adhered between apair of electrically insulating face sheets. In the embodiment of thepresent invention in which the shape memory alloy tendon is formed of anumber of shape memory alloy wires, tension is preferably applied to theshape memory alloy wires while the adhesive cures such that the shapememory alloy wires are maintained in a predetermined alignment, such asa predetermined spaced-apart and parallel alignment. In addition, theadhesive is preferably cured at reduced pressures in order to decreasevoid formation within the adhesive.

Once the shape memory alloy component has been formed, the shape memoryalloy component can be embedded within the composite material layers.While the composite material layers can be laid up and consolidatedaccording to conventional autoclave fabrication methods, oneadvantageous embodiment of the fabrication method forms the compositestructure according to a fiber placement process. According to thisembodiment, a number of ribbons of composite material are placed in aside-by-side manner on an underlying composite material layer. Theplurality of composite material ribbons are then consolidated to theunderlying composite material, such as by heating the composite materialribbons with a laser source.

According to one embodiment of the present invention, the shape memoryalloy component is preferably inserted between a pair of compositematerial ribbons during the fiber placement process. According toanother embodiment of the present invention, the composite materialribbons can be placed on the underlying composite material layer so asto define a trough between a pair of adjacent composite materialribbons. The shape memory alloy component can then be inserted withinthe trough defined between the pair of adjacent composite materialribbons.

Regardless of the manner in which the shape memory alloy component isinserted, additional ribbons of composite material can then be placed ina side-by-side manner on the shape memory alloy component. Preferably,the shape memory alloy component extends in a first predetermineddirection and the additional ribbons of composite material which areplaced on the shape memory alloy component extend in a secondpredetermined direction, different than the first predetermineddirection. Thus, during the subsequent consolidation of the overlyingcomposite material layers, the embedded shape memory alloy component isat least somewhat protected from the elevated temperatures required forconsolidation.

Accordingly, the composite structure of the present invention includesan embedded shape memory alloy tendon which is electrically andthermally isolated from the remainder of the composite structure toprovide controlled structural deformation of predetermined portions ofthe composite structure. In addition, the fabrication method of thepresent invention allows the shape memory alloy component to be readilyembedded within a composite structure, such as during a fiber placementprocess, without actuating or otherwise damaging the shape memory alloycomponent and without impairing the structural integrity or strength ofthe composite structure. Thus, a structurally deformable compositestructure can be efficiently fabricated in a manufacturing environmentaccording to the fabrication method of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a shape memory alloy component of oneembodiment of the present invention in which the shape memory alloytendon includes a number of shape memory alloy wires disposed between apair of face sheets.

FIG. 2 is a cross-sectional view of the shape memory alloy component ofFIG. 1 taken along line 2--2 and illustrating the adhesion of the facesheets to the shape memory alloy wires.

FIG. 3 is a cross-sectional view of a shape memory alloy component ofanother embodiment of the present invention in which the shape memoryalloy tendon is a shape memory alloy foil adhered between a pair of facesheets.

FIG. 4 is a perspective view illustrating the formation of a shapememory alloy component of one embodiment of the present invention inwhich tension is applied to a number of shape memory alloy wires.

FIG. 5A is a cross-sectional view of a composite structure according tothe present invention which includes an embedded shape memory alloycomponent.

FIG. 5B is an enlarged cross-sectional view of the composite structureof FIG. 5A taken along lines 5B--5B and illustrating the relativeorientations of the plurality of composite material layers.

FIG. 5C is an enlarged cross-sectional view of the composite structureof FIG. 5A taken along lines 5C--5C and illustrating the orientation ofthe shape memory alloy component relative to the surrounding compositematerial layers.

FIG. 6 is a perspective view illustrating the placement of a pluralityof ribbons of a composite material upon an underlying composite materiallayer during a fiber placement process according to one embodiment ofthe present invention.

FIG. 7 is a perspective view illustrating the insertion of a shapememory alloy component of the present invention within a trough definedbetween a pair of adjacent composite material ribbons according to oneembodiment of the fabrication method of the present invention.

FIG. 8 is a perspective view of a portion of a composite structureaccording to the present invention illustrating the surface egress ofthe shape memory alloy component of the present invention from theresulting composite structure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which a preferred embodimentof the invention is shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein; rather, this embodiment is provided sothat this disclosure will be thorough and complete and will fully conveythe scope of the invention to those skilled in the art. In the drawings,the thickness of the respective layers are exaggerated for clarity. Likenumbers refer to like elements throughout.

Referring now to FIG. 1, a shape memory alloy component 12 according tothe present invention is illustrated. One common type of shape memoryalloy component 12 known by those of skill in the art is a shape memoryalloy tow or actuator. The shape memory alloy component 12 comprises ashape memory alloy tendon 15 disposed between a pair of electricallyinsulating face sheets 20, 22. As best illustrated by FIG. 8, the shapememory alloy component 12 is preferably embedded within a number ofcomposite material layers 11 such that the shape memory alloy tendon 15can be externally actuated. By actuating the shape memory alloy tendon15, such as by raising the temperature of the shape memory alloy tendon15 above a predetermined transition temperature, the shape memory alloytendon 15 contracts to create a controlled structural deformation ofboth the shape memory alloy component 12 and at least a portion of theresulting composite structure 10.

The shape memory alloy tendon 15 is predominantly comprised of Nickeland Titanium but can include of a number of other elements, such asCopper, Zinc and Aluminum. It is the ratio of Nickel to Titanium presentin the alloy that primarily determines the transition temperature. Theshape memory alloy tendon 15 must be trained to the desired "contracted"shape. This training or conditioning of the shape memory alloy tendon 15can typically be performed in one of two ways by mechanically stretchingor prestraining the shape memory alloy tendon 15 as illustrated by FIG.4, or by heat treating. Although the amount of Nickel and Titanium inthe alloy primarily controls the resultant transition temperature, as isknown to those of skill in the art, the amount and type of conditioningperformed on the shape memory alloy tendon 15 also impacts the ultimatetransition temperature. In this regard, shape memory alloy iscommercially available from sources such as Shape Memory Applications,Inc. of Santa Clara, Calif. This source also provides kits which helpestablish how the material responds to conditioning.

Typically, the shape memory alloy tendon 15 is in a martensitic or"relaxed" crystalline state at ambient temperature and when heated abovethe transition temperature reverts to an austenitic or "contracted"state. As known to those skilled in the art, the relative difference inshape of a shape memory alloy between its relaxed state and itscontracted state is around 4%-8% and the shape memory alloy can beprestrained in either a uniform or segmented manner. The segmentedconditioning allows the shape memory alloy component 12 to be embeddedto deform only a certain area. As such, this segmented approachconcentrates the deformation effect into a desired area and allows for amore sophisticated deformation pattern.

As also known to those skilled in the art, the transition temperature asdiscussed above is typically a function of the composition of the shapememory alloy material as well as the conditioning performed on the shapememory alloy tendon 15, with transition temperatures ranging fromambient temperatures to both above and below ambient temperatures.However, it will be appreciated that the composition of the shape memoryalloy should preferably be selected such that the transition temperatureis high enough that the shape memory alloy will not be actuated undernormal operating or fabrication conditions, but will, instead, requireadditional heating in order to be actuated. Therefore, while it isunderstood that transition temperatures can be any number oftemperatures, it is preferred that the shape memory alloy composition besuch that the transition temperature is from about 100°-250° F., andmore preferably that the austenite finish temperature be from about200°-250° F.

As illustrated by FIGS. 1, 2, and 3, the shape memory alloy tendon 15can have a variety of configurations such as a plurality of shape memoryalloy wires 16 or a shape memory alloy foil 17. Although the shapememory alloy tendon of FIGS. 1 and 2 could have any number of shapememory alloy wires, one embodiment of the shape memory alloy tendonincludes between 3 and 10 wires. Regardless of the particularconfiguration, however, the shape memory alloy tendon 15 is disposedbetween a pair of electrically insulating face sheets 20, 22. Theelectrically insulating first and second face sheets 20, 22 arepreferably comprised of a thermoplastic material such aspolyetheretherketone (PEEK) or polyetherimide in order to effectivelyisolate the shape memory alloy tendon 15 from the remainder of thecomposite structure 10. Regardless of the material, the face sheetspreferably provide not only electrical isolation, but also thermalisolation in order to prevent actuation of other shape memory alloycomponents within the composite structure upon heating of one particularshape memory alloy tendon.

As illustrated in FIGS. 1, 2 and 3, the first and second face sheets 20,22 are adhered to opposite sides of the shape memory alloy tendon 15 toeffectively sandwich and contain the shape memory alloy tendon 15 (16,17) therebetween. The face sheets 20, 22 are bonded to the shape alloymemory tendon 15 with an adhesive 24. Although any number of adhesiveswould provide the necessary bonding, it is preferred that the adhesivebe curable at or around room temperature. For example, the adhesive canbe a two-part urethane.

In order to further isolate the shape memory alloy tendon and to preventsecondary actuation of other shape memory alloy tendons, the adhesive isalso preferably electrically and thermally insulating. In the embodimentof FIGS. 1 and 2, the shape memory alloy wires are preferably separatedby the adhesive such that each shape memory alloy wire can beindividually actuated without also heating and therefore actuating othershape memory alloy wires of the shape memory alloy tendon. In addition,the layer of adhesive between the shape memory alloy tendon and the facesheets further isolates the shape memory alloy tendon from thesurrounding composite material. One acceptable commercially availableadhesive is Ciba Geigy Uralane 5774® adhesive.

As best illustrated in FIG. 1, the face sheets 20, 22 have opposed endedges 12a and opposed side edges 12b. In addition, the shape memoryalloy tendon 15 extends outwardly from the opposed end edges 12a so asto provide external access for electrical connection or heating duringactuation of the shape memory alloy tendon. In contrast, it is preferredthat the shape memory alloy component 12 tendon be encapsulated with anadhesive 24 along the opposed side edges 12b so as to further insulatethe shape memory alloy tendon 15 from the composite structure and anyadjacent shape memory alloy tendons, thereby preventing any electricalcontact (or shorts) along the length of the shape memory alloy tendon15. This encapsulating adhesive is particularly advantageous as thelength of the shape memory alloy component 12 increases since anelongated shape memory alloy tendon, such as a shape memory alloy wire,can be somewhat difficult to align and the side edges of the shapememory alloy tendon may otherwise contact the adjacent composite layers.

In order to fabricate the shape memory alloy component 12, a first facesheet 20 is positioned on a support which is typically disposed within avacuum chamber or is capable of pulling a vacuum. The shape memory alloytendon 15 is then positioned over the first face sheet 20. The adhesive,typically a two part Urethane, is mixed and lathered over the shapememory alloy tendon 15 such that the adhesive 24 makes good contact withboth the first face sheet and the shape memory alloy tendon 15. Thesecond face sheet 22 is subsequently positioned over and adhered to theshape memory alloy tendon 15 so as to sandwich the shape memory alloytendon 15 therebetween. The shape memory alloy component assembly isthen cured at room temperature for a predetermined period of time, suchas 20 minutes. The shape memory alloy component assembly is preferablycured at increased pressure in order to minimize the number and size ofvoids within the shape memory alloy component 12. For example, one canincrease the pressure on the adhesive by placing a vacuum bag over theshape memory alloy component 12 and reducing the internal pressurethereby building the pressure upon the shape memory alloy component toup to about 1 atm. The increased pressure can also be provided withincreased weight, i.e., mechanically applied with external compressionsuch as with the use of hydraulics.

In order to insure that the shape memory alloy wires of the embodimentof FIGS. 1 and 2 do not contact one another, the alignment of theplurality of shape memory alloy wires 16 in a generally spaced apart andparallel arrangement is preferably maintained during the assembly andcure process. Therefore, as illustrated in FIG. 4, it is preferred thattension be applied to the shape memory alloy wires 16 during theassembly and curing process. For example, the tension can be provided bya tension and alignment tool 50. As illustrated, the tool providesclamps 52, 53 for engaging the opposing ends of the shape memory alloywires. As described below, at least one of the clamps is preferablymovable relative to an underlying platform 55. The tool 50 can alsoinclude a tension tightening means such as a fixed block 57 which issecured to the platform and a threaded connector or screw 58 whichinterconnects the movable clamp and the fixed block. By advancing theconnector, the spacing between the fixed block and the movable clamp canbe adjusted, thereby also adjusting the tension on the wires. However,other means of providing tension to the shape memory alloy wires duringthe fabrication process can be employed without departing from thespirit and scope of the present invention.

The shape memory alloy component 12 can have various shapes and sizes.However, in one advantageous embodiment of the present invention, theshape memory alloy component 12 is an elongated ribbon. For example theshape memory alloy component may be 8 feet long×0.25 inches wide×0.020inches thick. In addition, in the embodiment of FIGS. 1 and 2, the shapememory alloy wires can have various lengths and diameters. However, theshape memory alloy wires of one embodiment have a diameter of betweenabout 0.001 inches and about 0.250 inches and, more preferably, betweenabout 0.005 inches and about 0.010 inches.

As best illustrated in FIG. 5A, it is also preferable that the width ofthe shape memory alloy component 12 be generally the same as the widthof the ribbons or plies of composite material which surround theembedded shape memory alloy component 12. As described below, it is alsopreferable that the shape memory alloy component 12 have the same depthor thickness as the plies or ribbons of composite material in which theshape memory alloy component is embedded. By sizing the shape memoryalloy component and the plies or ribbons of composite material similarlyand, more preferably, identically, the fabrication process is simplifiedby providing a tight fit between the shape memory alloy component andthe surrounding plies or ribbons of composite material, therebyminimizing voids within the resulting composite structure. Additionally,providing a trough 28 which is deep enough to keep the shape memoryalloy component 12 flush with the adjacent layers of ribbon 25 will helpminimize stress placed on the shape memory alloy component 12 upon fiberplacement pressure application. Accordingly, the structural integrityand strength of the composite structure is enhanced. Although it ispreferable that the dimensions of the shape memory alloy component 12 beapproximately equivalent to the adjacent composite material ribbons, itwill be appreciated by those of skill in the art that the size of theshape memory alloy component 12 is not limited thereto and can be anynumber of desirable sizes with the only limitation being thatexcessively large shape memory alloy components may weaken the resultingcomposite structure.

Generally described, and as best illustrated by FIG. 6, the compositestructure is fabricated by a process which heats and consolidates theresin. Although the specification will primarily describe thefabrication method of the present invention in the context of a fiberplacement process, it will be understood by those of skill in the artthat the process of embedding the shape memory alloy component of thepresent invention within a composite structure can be performed by avariety of other composite fabrication process, such as a conventionalautoclave curing process in which a number of stacked plies are heatedat reduced pressures to consolidate the plies. Regardless of thefabrication method, however, the shape memory alloy component ispreferably sufficiently thermally isolated such that the heat employedduring the consolidation of the composite plies or ribbons does notactuate the embedded shape memory alloy component, as described in moredetail hereinafter.

In the fiber placement embodiment of the present invention, a ribbon ofcomposite material is initially laid down on a tool, such as acylindrical mandrel. Thereafter, additional ribbons of compositematerial are laid down on an underlying composite material layer.Typically, the composite material ribbon is drawn from a spool or creelto a nit point at which the composite material ribbon contacts theunderlying composite material layer. The composite material is heatedand consolidated at the nit point 60. Typically, a laser heat source 30is focused at the nit point 60 to heat and flow the resin in a polymermelt region. A compaction roller 62 simultaneously compacts the heatedcomposite material and provides pressure at the nit point 60 in thepolymer melt region. This pressure and heat bonds the underlyingcomposite material layer 11 with the ribbon of composite material 25 atthe nit point 60. While the placement of a single ribbon of compositematerial is described above, a number of ribbons of composite material,such as four ribbons as shown in FIG. 6, are generally placedsimultaneously on the substrate in a side-by-side manner so as toexpedite the fabrication process. The plurality of ribbons of compositematerial are then heated and consolidated as described above.

The composite material ribbons 25 are built up to create a plurality ofcomposite material layers 11. Thus, the composite structure 10 of thepresent invention typically includes a plurality of composite materiallayers 11 and an intermediate layer which includes at least one embeddedshape memory alloy component 12. As shown, it is preferred that theshape memory alloy component 12 be placed inward of the outer compositematerial layers 11 so as to protect and thermally insulate the shapememory alloy component from the environment.

In order to embed the shape memory alloy component within the compositestructure, the shape memory alloy component can be placed in a troughformed within a composite material layer. Preferably, a trough 28 isdefined between a pair of adjacent composite material ribbons 25a, 25band has approximately the same size and shape as the composite materialribbons 25 themselves. For example, during the fiber placement process,a ribbon of composite material 25 can be omitted or skipped to create atrough within the respective composite material layer. The shape memoryalloy component 12 could then be laid down or inserted within thistrough 28. Since the shape memory alloy component 12 is preferably thesame size as the ribbons of composite material 25, the shape memoryalloy component 12 will snugly and completely fill the trough 28 suchthat the structural integrity and strength of the resulting compositestructure is not impaired.

Alternatively, in the embodiment in which a plurality of ribbons ofcomposite material are simultaneously placed on the underlying compositelayers, one of the ribbons of composite material can be replaced by theshape memory alloy component such that the shape memory alloy componentis inserted between a pair of composite material ribbons, therebyfurther expediting the fabrication process of the present invention.Still further, a trough 28 can be routed into a consolidated plie(s) ofcomposite material layer 11. Regardless of the manner in which thetrough is formed, it is preferred that the trough 28 be sized to matchthe size of the shape memory alloy component as best illustrated by FIG.7. Thus, the shape memory alloy component will completely fill thetrough without extending above the adjacent ribbons of compositematerial 25a, 25b to thereby minimize any dimensional mismatch of theshape memory alloy component to the surrounding composite material.

The typical width of composite material ribbon 25 is in the range of0.240-0.250 inches. However, this width is only dictated by a guide barcurrently employed in the fabrication tooling and could easily be variedto 0.1-0.5 inches or even more. It will be appreciated, therefore, thatneither the width of the composite ribbon 25 nor the shape memory alloycomponent 12 is a limitation of the invention and composite ribbons ofany number of widths would perform equivalently. Comparably, the shapememory alloy component 12 likewise has an unlimited number of widths andwhile preferred, does not have to match the composite ribbon width. Itis envisioned that foil tendons 17 could easily function up to one footin width depending on the application.

Although the shape memory alloy component preferably has a size whichmatches the ribbons of composite material, the shape memory alloycomponent can be larger that the ribbon of composite material, such asthicker or wider or both. In this embodiment, a larger trough ispreferably defined to receive the shape memory alloy tow. For example,two or more adjacent ribbons of composite material can be omitted tocreate a wider trough. Alternatively, multiple layers of compositematerial can be built up which each define a trough at the same relativeposition to create a deeper trough. For example, as illustrated in FIG.5C, the shape memory component 12 has a thickness approximately equal totwo composite ribbons of material. In any event, the trough ispreferably sized to match the shape memory alloy component to minimizevoids or other structural irregularities within the composite material.

Once the shape memory alloy component has been inserted between a pairof ribbons of composite material, the composite structure 10 iscompleted by adding additional composite layers 11 over the shape memoryalloy component 12 to embed the shape memory alloy component 12 withinthe plurality of composite material layers 11. The shape memory alloytendon 15 is preferably thermally insulated such that it not be heatedabove its transition temperature (at least along a major portion of thelength of the shape memory alloy component 12) during the fabricationprocess. As will be understood by those of skill in the art, heating ofthe shape memory alloy tendon above the transition temperature willcause the shape memory alloy component tendon 15 to contract, therebypotentially deforming the surrounding composite structure. The shapememory alloy tendon is typically thermally insulated by a combination ofthe face sheets and the adhesive. However, the shape memory alloycomponent 12 can also be cooled to preclude actuation during processingand assembly of the composite structure.

As noted above, it is not uncommon for the nit area 60 to be exposed tohigh temperatures (above 1200° F.) and high pressures (at least 200psi). Thus, in order to further protect the shape memory alloycomponent, the shape memory alloy component 12 preferably does notextend above the surface of the neighboring ribbons of compositematerial such that the shape memory alloy component is not exposed toundue temperatures and pressures during the consolidation of subsequentcomposite material layers 11. Thus, in embodiments of the fabricationprocess of the present invention in which the shape memory alloycomponent is place in a trough, it is desirable that the trough beappropriately sized to match the size of the shape memory alloycomponent as described above.

In order to further protect the shape memory alloy component, it is alsopreferable that the composite material layer 11 immediately overlyingthe shape memory alloy component 12 be placed in a different orientationthan the orientation along which the shape memory alloy component 12extends. As will be appreciated by those of skill in the art, byoffsetting the placement orientations of the shape memory alloycomponent and the overlying ribbons of composite material, only apartial segment of the shape memory alloy component 12 will be heated atone time during the process of consolidating the overlying compositematerial layers, thereby preventing actuation of the shape memory alloycomponent 12 along the length.

The plurality of composite material layers 11 are also preferably laiddown in at least two different orientations in order to enhance thestrength and integrity of the resulting composite structure. As bestillustrated by FIGS. 5B and 5C, the composite layers 11 can be placed ineither a 0° or 90° orientation. In this embodiment, the 90° orientation(as indicated by left leaning hatch marks) indicates that the respectiveribbon of composite material extends in a generally helical loop aroundthe mandrel. In contrast, the 0° orientation (as indicated by rightleaning hatch marks) indicates that the respective ribbon of compositematerial extends in a generally axial direction along the mandrel. Thisintermingling of ribbon orientations advantageously provides additionalequilateral strength to the resulting composite structure. For example,a large complex composite structure which is formed by compositematerial layers which all extend in the 90° orientation would have atendency to warp in an uncontrolled and undesirable inward mannerdepending on the structure's shape and heat treat process parameters.Likewise, a composite structure which is formed by composite materiallayers which all extend in the 0° orientation would have a tendency towarp in an uncontrolled outward manner. Referring to the exemplaryembodiment of FIG. 5B, the composite layers 11 extend in 90°, 90°, 0°,0°, 90°, 0°, 0°, 90°, and 90° orientations, respectively.

As illustrated by FIGS. 5A and 5C, one embodiment of a compositestructure 10 comprises a 9-ply carbon reinforced composite materiallayers 11 with at least one shape memory alloy component 12 embeddedtherein. Also as illustrated, the shape memory alloy component 12 isplaced in a trough 28 which is both inboard of the ends of the compositestructure 10 and intermediate of the outer and inner layers of theplurality of composite layers 11. As such, the shape memory alloycomponent of this embodiment of the present invention is protected fromthe environment. As also illustrated by FIG. 8, it is quite possible andeven preferred to use multiple shape memory alloy components 12 within acomposite layer 11 of a composite structure 10. By providing a pluralityof embedded shape memory alloy components, the reliability of theresulting composite structure is enhanced since if one shape memoryalloy component 12 fails, the other surrounding shape memory alloycomponent(s) 12 could still be actuated to create the desired controlledstructural deformation. It is also possible to place the shape alloymemory component 12 along only a portion of the length of the trough 28and to fill the remainder with a ribbon of composite material 25 or touse segmented shape memory alloy tendons 15. As will be appreciated bythose of skill in the art, the shape memory alloy components 12 can beplaced and configured in a variety of manners with the primarylimitation being dictated by the intended function and geometry of theresulting composite structure.

Even though the shape memory alloy component is disposed within anintermediate and therefore interior portion of the composite structure,the shape memory alloy component 12 is preferably externally accessiblesuch that the shape memory alloy tendon can be actuated, such as bydirecting electrical current through the shape memory alloy tendon whichheats and therefore actuates the shape memory alloy tendon. Preferably,the external access to the shape memory alloy component is provided viaa surface egress since. As best illustrated by FIG. 8, the shape memoryalloy component 12 can be egressed through a number of aligned openingsdefined by the underlying composite material layers 11 and through asurface egress aperture 42 in the fabrication mandrel or tool 40 whichis aligned with the openings in the underlying composite materiallayers. The openings in the underlying composite layers can be createdby routing or drilling openings with the use of a guidebar which arealigned with a previously defined slot or aperture 42 in the fabricationmandrel 40. By surface egressing the shape memory alloy component, theshape memory alloy component is readily accessible for externalactuation, such as electrical resistance heating.

As described above, the fabrication method of the present invention willenable the shape memory alloy component(s) 12 to be placed in any numberof directional orientations within the composite structure 10, such astransverse, longitudinal, or helical orientations. Upon actuation, theembedded shape memory alloy component(s) 12 then contracts to performthe desired structural deformation. For example, actuation of the shapememory alloy component can twist, warp, displace and or stiffen thecomposite structure. As will be apparent to those skilled in the art, acomposite structure incorporating the shape memory alloy component ofthe present invention can be employed in a variety of applications. Forexample, the composite structure can be employed in smart structureapplications, such as composite wings, engine inlets and outlets, andother control surfaces. As a result of the weight savings andperformance advantages provided by the composite structure of thepresent invention, the operating range of an aircraft which includes avariable engine inlet geometry should be extended. As another example,the composite structure of the present invention can form an enginepedestal which is controllably deformed to dampen the vibrations of theengine.

As described above, the composite structure of the present inventionincludes an embedded shape memory alloy tendon which is electrically andthermally isolated from the remainder of the composite structure toprovide controlled structural deformation of predetermined portions ofthe composite structure. In addition, the fabrication method of thepresent invention allows the shape memory alloy component to be readilyembedded within a composite structure, such as during a fiber placementprocess, without actuating or otherwise damaging the shape memory alloycomponent and without impairing the structural integrity or strength ofthe composite structure. Thus, a structurally deformable compositestructure can be efficiently fabricated in a manufacturing environmentaccording to the fabrication method of the present invention.

In the drawings and the specification, there has been set forth apreferred embodiment of the invention and, although specific terms areemployed, the terms are used in a generic and descriptive sense only andnot for purpose of limitation, the scope of the invention being setforth in the following claims.

That which is claimed is:
 1. A method of fabricating a compositestructure adapted for controlled structural deformation, the methodcomprising the steps of:forming a shape memory alloy component, whereinsaid forming step comprises the step of adhering a shape memory alloytendon between a pair of electrically insulating face sheets, andwherein the shape memory alloy tendon has a relaxed shape attemperatures below a predetermined transition temperature and acontracted shape at temperatures above the predetermined transitiontemperature; embedding the shape memory alloy component within aplurality of composite material layers such that the shape memory alloytendon is electrically isolated from the surrounding composite materiallayers; and establishing electrical communication with the shape memoryalloy tendon of the embedded shape memory alloy component such thatsubsequent actuation of the shape memory alloy tendon by raising thetemperature of the shape memory alloy tendon above the predeterminedtransition temperature creates a controlled structural deformation ofboth the embedded shape memory alloy component and the surroundingcomposite material layers.
 2. A method according to claim 1 wherein theshape memory alloy tendon is a plurality of shape memory alloy wires,and wherein said forming step further comprises the step of applyingtension to the plurality of shape memory alloy wires during saidadhering step such that the shape memory alloy wires are maintained in apredetermined alignment.
 3. A method according to claim 1 wherein saidforming step further comprises the steps of:disposing the shape memoryalloy tendon on a first electrically insulating face sheet; applyingadhesive to the shape memory alloy tendon and the first face sheet;disposing a second electrically insulating face sheet on the shapememory alloy tendon, opposite the first face sheet; and curing theadhesive such that the shape memory alloy tendon is adhered between thefirst and second face sheets.
 4. A method according to claim 3 whereinsaid forming step further comprises the step of increasing the pressureon the shape memory alloy component during said curing step to reducevoid formation within the cured adhesive.
 5. A method according to claim1 wherein said embedding step comprises:placing a plurality of ribbonsof composite material in a side-by-side manner on an underlyingcomposite material layer; and consolidating the plurality of compositematerial ribbons to the underlying composite material layer.
 6. A methodaccording to claim 5 wherein said forming step comprises forming theshape memory alloy component in ribbon form, and wherein said embeddingstep further comprises the steps of:inserting the shape memory alloycomponent between a pair of composite material ribbons; placingadditional ribbons of composite material in a side-by-side manner on theinserted shape memory alloy component; and consolidating the additionalcomposite material ribbons to thereby embed the shape memory alloycomponent within the resulting composite structure.
 7. A methodaccording to claim 6 wherein said inserting step comprises inserting theshape memory alloy component such that the shape memory alloy componentextends in a first predetermined direction, and wherein said step ofplacing additional composite material ribbons on the shape memory alloycomponent comprises placing additional composite material ribbons on theinserted shape memory alloy component such that the additional compositematerial ribbons extend in a second predetermined direction, differentfrom the first predetermined direction.
 8. A method according to claim 5wherein said forming step comprises forming the shape memory alloycomponent in ribbon form, wherein said placing step comprises the stepof defining a trough between a pair of composite material ribbons, andwherein said embedding step further comprises the steps of:inserting theshape memory alloy component within the trough defined between the pairof composite material ribbons; placing additional ribbons of compositematerial in a side-by-side manner on the inserted shape memory alloycomponent; and consolidating the additional composite material ribbonsto thereby embed the shape memory alloy component within the resultingcomposite structure.
 9. A method according to claim 8 wherein saidinserting step comprises inserting the shape memory alloy componentwithin the trough such that the shape memory alloy component extends ina first predetermined direction, and wherein said step of placingadditional composite material ribbons on the shape memory alloycomponent comprises placing additional composite material ribbons on theinserted shape memory alloy component such that the additional compositematerial ribbons extend in a second predetermined direction, differentfrom the first predetermined direction.
 10. A method according to claim5 wherein said consolidating step comprises heating the plurality ofcomposite material ribbons with a laser source as the composite materialribbons contact the underlying composite material layer to thereby bondthe composite material ribbons to the underlying composite materiallayer.